The development of data processing techniques in recent years has prompted studies on the extraction of significant information from among earth observation data (to be referred to simply as observation data hereinafter) acquired by satellites, airplanes, and the like (to be referred to as flying objects hereinafter as appropriate) and the use of such information for disaster prevention, resource exploration, and the like.
At the same time, the improvement in the power of resolution of earth observation sensors and the growing amount of observation data have led to an increased demand for greater transmission capacity for transmitting acquired observation data to the ground.
Methods for sending observation data to the ground can be roughly grouped into those in which a flying object communicates directly with a terrestrial station and those in which a flying object transmits data to a terrestrial station via a geostationary orbit satellite, which is accessible at all times from the terrestrial station. In the direct communication methods, however, the communication between a flying object and a terrestrial station is possible only when the terrestrial station can be directly seen from the terrestrial station as the flying object travels at a high speed.
In any of the methods, however, available bands for microwave communication are heavily limited, which is disadvantageous. Thus, optical space communication systems are drawing attention as they are not bound by band restriction and can be operated with greater capacity.
For fiber-optic communication systems, when looked at from such a viewpoint, digital coherent technology, which combines coherent detection and digital signal processing, is worthy of attention. The coherent detection technology features a high reception sensitivity achieved in a phase shift keying method by the interference of very weak received optical signals with local oscillator signals.
However, as the flying object travels at a high speed, a Doppler shift occurs in the signal light. Thus, to achieve a high sensitivity reception with a phase shift keying method, it is necessary to remove the effect of Doppler shift. In other words, the range of compensation for the relative frequency difference between the carrier frequency of the signal light and the frequency of the local oscillator signal has to be expanded to the amount of Doppler shift that occurs.
With respect to the digital coherent technology, several techniques have been proposed for compensating for the relative frequency difference between the carrier frequency of the signal light and the frequency of the local oscillator signal (i.e., frequency offset) by digital signal processing.
According to NPL 1, for example, with respect to a QPSK (quadrature phase shift keying) signal, the phase difference between two consecutive samples received is raised to the power of four to remove the effects of data modulation and then averaged to reduce the effect of noise. Then the phase difference between the consecutive samples is calculated, which corresponds to the frequency offset.
NPL 2 proposes a decision-directed phase locked loop (DDPLL). The DDPLL calculates a phase error by initially performing a symbol decision with respect to the received sample and performs feedback control based on the phase error.
FIG. 16 is a common block diagram of such a DDPLL. The DDPLL 100 includes a phase adjustment device 101, a phase error calculation circuit 102, a loop filter 103, and a voltage-controlled oscillator (VCO) 104.
The phase error calculation circuit 102 includes a symbol decision unit 102a, a complex conjugate unit 102b, a multiplication device 102c, and an argument calculation unit 102d. 
A 1-sample/symbol signal (sample) G1 sampled at the timing of a symbol center is inputted to the phase adjustment device 101, phase-adjusted and outputted by the phase adjustment device 101, and inputted to the phase error calculation circuit 102.
The phase error calculation circuit 102 selects a most probable symbol decision candidate for each sample and the complex conjugate unit 102b calculates a complex conjugate for the result of the selection. The complex conjugate so calculated is multiplied by the original sample in the multiplication device 102c and the argument calculation unit 102d calculates an argument. This argument is inputted to the phase adjustment device 101 via the loop filter 103 and the VCO 104, as a phase compensation signal G2.
The phase adjustment device 101 compensates for the Doppler shift contained in the sample using the phase compensation signal G2.